Zircon U-Pb dating and geochemistry of the volcanic rocks from Maoniushan Formation in Amunike area, Qinghai Province, and its geological implications

Geochemical analysis and LA-ICP-MS zircon U-Pb dating were conducted for zircon grains and geochemical characteristics of the volcanic rocks from Maoniushan Formation in Amunike area. The geochemical characteristics show that the volcanic rocks form Maoniushan Formation have the following data: SiO₂=53.94%~64.69%, total alkali content(ALK(Na₂O+K₂O))=5.38%~ 9.86%, varying in a large range, Rittmann Index(δ)=1.45~4.54, belonging to alkaline rocks and calc-alkaline rocks, and all of aluminum saturation indexes A/CNK are higher than 1.1, exhibiting characteristics of S type granite. Rare earth element analyses reveal that ΣREE=68×10^-6~372×10^-6,and LREE/HREE=4.51~18.25, exhibiting right-dipping REE patterns strongly enriched in LREE and depleted in HREE. Trace elements analyses reveal strong enrichment of incompatible elements Rb, Ba, Th and conspicuous depletion of Ta, Nb, assuming the curve form that cobweb curve appears frequently in peak and valley. CL investigations show that zircon grains from Maoniushan Formation are euhedral in shape and display fine-scale oscillatory zoning as well as Th/U ratio of 0.4~ 1.3, implying their magmatic origin. Zircon dating of two samples indicates ²⁰⁶Pb/²³⁸U age of 371~399Ma, weighted mean ²⁰⁶Pb/²³⁸U ages of 374.8±8.1Ma and 392.8±3.3Ma, which are interpreted to be the time of the formation of volcanic rocks. Therefore, it is suggested that the Maoniushan Formation should occur in the Middle-Late Devonian period. Combined with regional characteristics, it is concluded that the formation of volcanic rocks in Maoniushan Formation was related to the evolution of the northern area of the Qaidam Basin, and volcanic rocks were probably formed by lithospheric extension at the post-collision stage.